Electric water heating apparatus



March 24, 1964 E- F. DILLON ELECTRIC WATER HEATING APPARATUS Filed Dec.50, 1960 FIG. 4 l3 INVENTOR. F. Dl LLON \ALJLM/ EUGENE BY W ATTORNEYtypes of heating unit.

United States Patent 3,126,470 ELECTRIC WATER HEATING APPARATUS EugeneFrancis Dillon, Chicago, Ill., assignor to General Electric Company, acorporation of New York Filed Dec. 30, 1960, Ser. No. 79,694 1 Claim.(Cl. 219-38) This invention relates to water heaters of the resistanceheater storage tank type.

Water heaters of this type usually utilize one of two The most commonheating unit is a metal sheathed immersion unit which is introduced intothe tank through a side wall opening. This arrangement is advantageousas respects direct heat transfer to the liquid but has the practicaloperating disadvantages of liming up in some of the harder waters and ofgalvanic erosion in installations in which magnesium rods or poweredanodes are used to protect the tank against corrosion. Suchinstallations require special electrical protection such as disclosed inDicome et al., 2,810,815. The second general form of heating unitcomprises the clamp on type as disclosed in Vogel et al., 2,452,214,issued October 26, 1948, and assigned to my present assignee. Obviously,the clamp on type, not being in direct contact with the water is notsubject to erosion or to the other weaknesses of the immersion units,but clamp on units are expensive to apply because of the harnessnecessary to maintain the heating element tightly against the tank wallto insure good heat transfer. Both of. the forms of resistance heatershave a common disadvantage: a rather cumbersome inventory of heatingunits of various wattages, or complete water heaters of variouswattages, is required to satisfy the difierent wattage ratingpreferences which are prevalent in various sections of the country. Anelectric utility, for example, may insist upon a specific wattage ratingin order that a large number of water heater installations may be madewithin the capabilities of its electric distribution system.

It has been previously proposed to coat the exterior of the storage tankor appropriate portions thereof, with electrical resistance films havingthe necessary wattage per square inch density, in lieu of the olderforms of heater units. These films are inexpensive as such, althoughthey require that the tank wall be coated with a satisfactory insulatingsubstrate and thus introduce a somewhat more costly and demandingmanufacturing procedure. There are, however, two real advantages: adirect heat path exists between the film and the tank wall thus insuringgood heat transfer with the external wall of the heater tank withoutclamping harnesses or the like, and it is possible by applying terminalbands at locations which will produce a range of wattages according tothe group of terminals utilized by the installation electrician, toproduce at very nominal extra cost a standard water heater which is ofitself adaptable to the preference or the specifications of a specificcustomer.

I have found, however, that as a result of practical mass productiontechniques, the resistance film may have minute deficiencies which aresubject to breakdown when the heater is in service. Such breakdowns maycompletely interrupt the electrical resistance circuits of which thefilm is the primary part, thus rendering useless that heating element ofthe water heater. It may happen at any time after installation andbecause there is no practical means of rectifying the failure in thefield, service can be restored only by a complete replacement of thewater heater. In the mass manufacture of water heaters embodyingresistance films it is a practical impossibility to insure absolutelyperfect films or insulating substrates. Voltage irregularities which maybe experienced after the heater is in actual service, may result incurrent concentrations ICC producing'hot spots which actually break downa small area of the resistance film. More importantly, I have found thatthe failures do not remain localized, but instead, spread laterallyalong a line which is substantially parallel to the line of the terminalstrips and about midway between them. A breakdown of the film at aminute area, therefore, appears inevitably to lead to the failure of theentire resistance element. It is my experience that it is most unusualfor there to be more than one or two of such danger points in any oneresistance film; whereupon I conceived that if means compatible withmass production practices were found to insure that any film failurewould be strictly localized, the matter of film failure could not sodisadvantageously affect the water heater performance as to require acomplete replacement of the water heater itself.

In a presently preferred form of carrying out the objective oflocalizing resistance film failures, I apply the necessary substrate tothe exterior wall of the tank and apply thereon a resistance film havingthe desired electrical resistance characteristics. The substrate andfilm will be applied to the tank wall in the form of relatively widebands which encircle the tank which as is well known, is usually ofcylindrical shape. After the film is deposited on the tank but beforethe terminal bands are applied thereto, I subdivide the film by means ofscribe lines which penetrate the film and usually penetrate at least theouter surface of the insulating substrate without exposing any of thebare metal of the tank wall. The scribe lines may be of the order of Aof an inch wide and will be axial in direction. The electric terminalbands are then applied along the appropriate marginal edges of theresistance film and terminal members applied thereto, whereupon when theterminal members are connected into an electric power circuit the filmactually becomes a series of individual resistances in a parallelelectrical circuit.

The scribe lines may be applied to the film by a simple tool of themilling machine type with respect to which the tank is rotated inindexing fashion to accomplish the required subdivision of the film. Ina fifty gallon capacity water heater manufactured pursuant to commoncorn mercial standards, the tank diameter is of the order of 20". I havefound that fifteen subdivisions will produce the desired insuranceagainst the failure of the heating element. It will be obvious that inthe event a hot spot failure develops in any of the subdivisions, thefault can migrate only to the marginal limits of the particularresistance area. Since the wattage of each of the resistance areas isonly a small portion of the total wattage an [individual failure or thefailure of two such resistance areas will not effect such a largereduction of the wattage capacity as would be noticeable in theperformance of the water heater.

Other features and advantages of the invention will be obvious from thefollowing detailed description of a typical form thereof read inconnection with the accompanying drawings in which:

FIG. 1 is a vertical elevation of an electric water heater embodying thepresent invention with the exterior casing of the heater shownfragmentarily and the number of heating units being reduced to two forsimplicity of representation. In this figure some of the scribed linesdelineating the electric resistance areas illustratively extend entirelyacross the areas and some extend from one terminal strip toward theother. These are optional arrangements and not intended in a limitingsense;

FIG. 2 is an enlarged section taken on lines 2- -2 of FIG. 1;

FIG. 3 is an enlarged section taken through a scribed area of a heatingunit on lines 3--3 of FIG. 1; i

FIG. 4 is 'a schematic representation of a method of scribing theheating units; and

FIG. is a schematic representation of a method of applying the terminalbands.

FIG. 1 shows a typical water heater tank 1 having the usual cold waterinlet 2 and hot water outlet 3. Tank may be of any suitable materialincluding cold rolled steel having any suitable lining (not shown) Onthe interior thereof for protection against water corrosion. Forexample, the steel in which the tank is made may have a lining ofnon-corrosive metal such as stainless steel, or of vitreous material.The lining material is unimportant as respects the present invention.The exterior wall of the tank is uncoated except for a thermallyconducting and electricallyinsulating substrate as presently described.Pursuant to conventional practice the tank may have an exterior shell 4enclosing the thermal insulation layer 5 which is usually of glass fiberor mineral wool. Such materials are thermally efiicient and of course,are electrically insulating as well.

The exterior of the tank is provided with heating elements embodying thepresent invention. Illustratively there are only two such elements,respectively 6 and 7, located in the upper and in the lower areas of thetank. The resistance elements are selected to provide a total wattagewhich is consistent with the requirements of a quick recovery waterheater. For example, the total wattage provided by the heating elementsmay be 4500. As previously indicated there may be any number ofindividual heating elements, or the entire surface of the tank may becoated with the resistance heating material and appropriate terminalarrangements provided so that only selected portions of the resistancematerial are active.

In the illustrated form, resistance material is of the film type appliedto an insulating substrate for the preven tion of short circuits betweenthe film and the tank.

The exterior of the tank is grit-blasted in preparation for theapplication of the substrate. The exterior surface must have a bright'finish and those portions to which the substrate is to be appliedshould not be handled after blasting. The tank is then conveyed to aspray booth within which the tank is rotated slowly on a vertical axiswhile being manually sprayed with substrate material. Any conventionalspray guns of the type commonly used for spraying porcelain frits may beused. The tank may be masked by conventional heat resistance mask strips(not shown) to delineate the substrate application.

The preferred substrate is one which will provide a hard, smooth, base,with minimum of voids, for the conductive film. Where the tank itself isof such construction or composition to withstand temperatures of theorder of 1540 to 1580 F. without deterioration of its corrosionresistantlining, a waterresistant porcelain enamel glass is an excellentsubstrate material. A typical formulation, based on one hundred parts(pounds) of glass frit, includes five parts of clay, onehalf part borax,from one-eighth to one-quarter part magnesium carbonate, from five toten parts of silica and from thirty-eight to forty-two parts of water.This material is fired in a conventional oven, at from 1540 to 1580 F.The thickness of the layer may be from five to ten mils. 7

When it is not possible to expose the tank to such temperatures, aphosphate bonded ceramic may be used.

A presently preferred phosphate bonded ceramic formula is as follows:

150 cc. of deionized water at room temperature 56 cc. of commercialgrade phosphoric-acid 30 grams of 400 mesh hydrated alumina 450 grams of400 mesh silica.

The ingredients are mixedin a ball mill for one-half hour. The mixtureis sprayed onto the surface of the tank at a rate of 72 grams per squarefoot. This amounts to 21 cc. per square foot, or a substrate thicknessof from ten to fifteen mils.

The tank is held at 600 F. until the phosphate material is hard and dry.

This material forms an excellent mechanical bond with the tank wall. Itmay be desirable to provide a light vitreous enamel dust coat, as iswell known in the art, on the tank Wall before applying the phosphatebonded ceramic, to limit the attack of the phosphoric acid. There arealso available, well known inhibitors which may be mixed with thephosphate bonded slip, to reduce the etching action of the acid. In itsdried or pseudo-vitreous state, the phosphate bonded ceramic is ratherporous, and does not have a hard, glassy surface to the desired degree.These deficiencies can be ameliorated in substantial measure by sprayingthe substrate with a filler material such as hydrolyzed ethyl silicateand burnishing the surface with a burnishing wheel or cylinder (notshown).

In any event, when the masks are removed, the tank has an uppersubstrate layer 8 and a lower substrate layer 9.

The deposit of the electrically conductive film 10 is done in an ovenheld at approximately 1050 F. The marginal edges of the substrate may bemasked by metal strips or the like before spraying with the filmmaterial.

The film material is prepared by mixing two preparations as follows:

Part 1 consists of a 55 cc. of proof ethyl alcohol containing 5% ofmethyl alcohol. This is mixed with 10 grams stannic chloride.

Part 2 consists of 20 grams of concentrated hydrochloric acid, mixedwith 5 grams of antimony trichloride. This mixture should be keptcovered at all times.

The spray mixture consists of 11 drops, or about .55 cc. of Part 2,added to the above quantity of Part 1. About one hour supply of solutionis the maximum that should be stored, and the container should bepolyethylene lined or equivalent. The spray guns should have carbidetips.

The mixture is applied by means of a spray gun for each of the twoheater elements 6 and 7. During the spraying, the tank is rotated on avertical axis at about 30 r.p.m. and means (not shown) are provided foruniformly moving the spray guns back and forth over the area to besprayed. The film material is deposited at the rate of about 5 cc. persquare foot, producing a finished film having a thickness on the orderof 5 microns. The technique of spraying is essentially conventional. Inview of the nature of the spray material spraying is accomplisheddirecting the spray through suitable apertures in the wall of the oven(not shown).

The tank is allowed to cool at a normal rate and is then removed fromthe oven. The masking bands (not shown) are removed, whereupon the areaof the resistance film is defined by the upper and lower marginal bands11 of uncoated substrate material.

The tank with its substrate and resistance film is then transferred toapparatus schematically shown in FIG. 4 on which the scribing operationis performed. The scribing tool 12 may be of a milling machine type.

The scribers or milling cutters 14, affixed to a reciprocating head 15,scribe the film and a portion of the substrate so that in none of thescribe areas is the bare metal of the tank exposed. FIG. 3 schematicallyillustrates the substrate condition at the termination of the scribingoperation. During the scribing operation the tank is indexed byappropriate rotation of the mounting shafts 13 to provide for thedesired number and spacing of the scribed lines. For simplicity of thisprocess, it is contemplated that the substrate bands will be scribedacross their full depth, although it will be obvious that it isnecessary only for the portion of the resistance layer between theterminal bands to be electrically interrupted. Preferably, the scribedline is of the order of in width.

After the scribing operation, the heating element areas are again maskedto cover the marginal areas 11 of the substrate and suflicient area ofthe resistance film to provide for the application of terminal bands inelectrical connection with the resistance film throughout the entirecircumference of the tank. Obviously, the total of the area of theresistance film between the terminal bands constitutes the effectiveheating unit area. As schematically shown in FIG. 5 the upper and lowermasking bands 16 cover the marginal areas 11 of the substrate, and themasking band 17 covers the resistance film to the extent providing thenecessary areas 18 of the resistance film. The terminal bands may beformed of aluminum sprayed on with a conventional gun 209 which atomizesan aluminum wire 21 and projects it under gas pressure against theexposed areas of the resistance film, as well understood in the art. Theresulting terminal bands 22 are about /2" in axial dimension and from.003" to .005 thick. The current carrying capacity of a terminal striphaving such dimension is adequate for the load to be applied thereto,which in each of the heating elements may be of the order of 2250 watts.

It will be noted in FIG. 1 that some of the scribed lines extendentirely across the film, whereas others extend from one or the other ofthe terminal bands but do not intersect both. Either or both of thesearrangements may be employed, although the full scribe is preferred. Itis considered essential that the scribe lines intersect each terminalband, whether or not the intersections are serial or alternate, and thateach of the scribe lines extend substantially beyond the central areasof the resistance films.

After the completion of the terminal strips the tank is provided withelectrical terminals. As schematically shown in FIG. 2 the terminals maycomprise carbon contacts 23 mounted on a steel stem 24 which is bondedto a rubber grommet 25. The grommet is mounted in a Z bracket 26 weldedor otherwise secured to the exterior of the tank and inherently springbiased to urge the shaft 24 in the direction of the terminal band 22. Asindicated schematically in FIG. 2, the distortion of the rubber grommet25 maintains the carbon contact 23 always in conductive relation withthe terminal strip 22.

As a final manufacturing step, the heating elements may be coated with asuitable electrically insulating water repellent film (not shown) toinsure against resistance changes which might otherwise result frommoisture or dampness. A satisfactory coating material comprises a clearalkyd resin varnish, such as manufactured and sold by applicants presentassignee under the trademark Glyptal.

The terminals provide for the connection of appropriate electrical powerleads 27 which may be arranged in any electrical circuit includingthermostats (not shown) as well known in the art. The entire heater tankassembly is thereupon covered with the electrical and thermal insulationmaterial 5 and enclosed in the protective and decorative outer cover 4.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of the invention.

I claim:

A water heater of the storage tank resistance heated type, comprising acylindrical metallic pressure vessel; a layer of vitreous electricallyinsulating and thermally conducting material fused to the exteriorsurface of said vessel to provide a continuous band extending about theperiphery thereof; a plurality of strips of electrically conductiveresistance material disposed on said band in good heat transfer relationtherewith, said strips comprising a film essentially of tin oxide and ofabout 5 microns thickness, each strip terminating short of therespective marginal edges of said insulating band and there being anelectrically insulating gap insulating each said strip from its adjacentstrip; first and second terminal bands of electrically conductivematerial extending continuously about said vessel inwardly of therespective marginal edges of said insulating band in electrical contactwith each of said strips; first and second terminal supporting bracketsfixed relative to said vessel adjacent the respective first and secondterminal bands; an electric terminal including a rod-like conductormounted in each of said brackets; annular resilient electric insulationmeans affixed to each said rod-like conductor and associated bracket andbiased to urge said terminal resiliently into contact with theass0ciated terminal band; means for connecting each said terminal to asource of electric energy; and a mass of thermally and electricallyinsulating material enclosing said pressure vessel and the above-namedelectrically conductive components thereof.

References Cited in the file of this patent UNITED STATES PATENTS1,771,273 Smith July 22, 1930 1,870,619 Flanzer Aug. 9, 1932 1,978,089Jones Oct. 23, 1934 2,452,214 Vogel et a1. Oct. 26, 1948 2,473,183Watson June 14, 1949 2,557,983 Linder June 26, 1951 2,629,166 Marsten etal Feb. 24, 1953 2,641,675 Hannahs June 9, 1953 2,678,990 Quirk May 18,1954 2,801,321 Prindle June 30, 1957 2,939,807 Needham June 7, 19602,942,331 Smiley June 28, 1960 3,004,130 Miller Oct. 10, 1961 3,026,400Van Sciver Mar. 20, 1962

